1. Field of the Invention
The present invention relates generally to steel torsional elements and methods for hardening, and more particularly to axles of the type that include an integral sun gear for a planetary final drive reduction, and to a method for hardening the sun gear and shaft portions of the axle to two different depths.
2. Description of the Prior Art
Off-highway mobile construction equipment, such as shovel loaders, rough terrain lift trucks, dozers and cranes, etc., usually use a planetary final drive reduction in each wheel; and the axles may include an integral sun gear that is forged and machined onto one end of the axle.
Because of the relatively small size of the sun gear in relation to the magnitude of transmitted torques, and because the sun gear transmits torque to a plurality of planet gears, the surface of the teeth of the sun gear must be hardened both for wear-resistance and for resistance to surface compressive fatigue. In addition, this type of application includes high shock loads, so that the teeth must have a tough core.
Thus, there is a requirement for a low carbon core that is hardened in excess of Rockwell C-60 for a depth of 0.040, 0.060, or perhaps 0.075 inches, depending upon the loading conditions; and there is a requirement for core ductility that precludes hardnesses above the Rockwell C-40 range.
Carburizing is a preferred method for achieving surface hardness of a sun gear because a low carbon alloy steel, such as SAE 4820-H provides good core strength, and the carburizing process increases the carbon content at the surface to about 0.90 percent carbon, providing a more wear-resistant surface than would be achieved by hardening a medium carbon steel to the same hardness.
It has been common practice to carburize the entire shaft along with the sun gear portion because of a resultant increased torsional strength in the shaft due to increased strength in the hardened case. For example, in a vehicle of the type previously specified, the axle can be up to three inches, or even larger, in diameter; and so the percentage increase in strength due to case hardening to a depth of 0.075 inches, will be small.
In addition, the formation of a martensitic case results in an increased volume of metal in the case, putting the case in compression, putting the core in tension, and resulting in high residual stresses at the interface between the case and the core. These stresses at the case-core interface combine with torsional stresses to cause metal fatigue failures at the case-core interface, thus reducing, or even obviating the strength improvement due to the increased strength of the hardened case.
Fatigue failures of torsionally loaded and case-hardened shafts typically start at the case-core interface, not only because of the aforementioned interface stresses, but also because the torsionally induced shear stresses in a shaft are directly proportional to the distance from the center of the shaft. Thus, the ratio of stress to shear strength is the highest at the radial location where the high shear strength of the case is replaced by the lower shear strength of the core.
Fatigue failures of case-hardened axles, or other torsional machine elements, show one of two types of characteristic failures. In one of the typical types of failures, the fatigued area is circumferentially disposed around the case-core interface and includes radially disposed cracks. The resultant reduction in the ability to linearly distribute stresses between the core and the case then results in complete transverse fracture of the axle.
In the other characteristic fatigue failure, a double crack starts at one point of the case-core interface and proceeds through the hardened case to the surface of the axle in a "V" shape. The failure in the hardened case then proceeds longitudinally along the surface of the axle, leaving a longitudinally disposed and diamond-shaped pattern of failure at the surface, and breaking out diamond-shaped pieces of the hardened case. The diamond-shaped pattern of the resultant case failure is typical; since brittle materials that are in shear fail in an oblique plane by tension.
In the first of the above-related characteristic failures, the failure is primarily a fatigue failure in the portion of the core that is radially proximal to the hardened case; whereas, in the second type of failure, inability of the core to sustain proportional distribution of stresses in the transverse plane, sometimes due to creep stresses rather than by actually exceeding the proportional limit, causes an increase in the stress in the hardened case with resultant fatigue fracturing of the hardened case.